Mundus istum M. En(n)ius in the manuscripts of Att. 15.26.5 is surely corrupt, as has been unanimously acknowledged (above all Cicero would avoid giving the three parts of a name in the order cognomen + praenomen + nomen, not to speak of the inexplicable istum). Also the modern Vulgate Mundus iste cum M. Ennio, introduced by Wesenberg in his Teubner text, is an improbable guess. Shackleton Bailey has recently proposed Maenius or Men(n)ius as the gentile name of Mundus. Mennius, however, is a very rare name and does not occur in Republican documents, while Maenius, although attested in Republican inscriptions, diverges unnecessarily from the manuscript tradition. Moreover, Shackleton Bailey must forcibly change istum to iste (even if he does not say so expressly). But it is possible to avoid practically any infringement of the transmitted text if we simply read Mundus Istummenius. The name (H) istumen(n)ius, (H)istimen(n)ius (also Inst-), written in a wide variety of ways, does occur some 20 times in urban inscriptions, mostly of the early Imperial period. It is attested also outside Rome: at Velitrae (CIL X 6556, of the early Imperial period), and even as far away as Gallia (CIL XIII 739, Bordeaux, early Empire). This gens must therefore have been somehow present among the Roman population of the Julio-Claudian age. In particular, attention should be paid to an Instumennius on a tessera nummularia of 60 B.C. (CIL I 915). No major figures occur in this gens, the name remaining restricted to the lower strata of the Roman population. That suits Mundus down to the ground. He clearly belongs to the grey mob of Rome. If he is, as it seems, identical with that Mundus mentioned in 15.29.1, then Cicero gives his family name in the first instance. The transmitted form with -mm- could represent a transposition of the double consonant of the common form Istumennius.